1. Field of the Invention
The present invention relates to a multilayer piezoelectric element constructed from a plurality of thin piezoelectric layers which are stacked with each other in a vertical direction, through which it can be obtained displacement in the vertical direction (stacked direction) when a voltage is applied thereto.
2. Description of Related Art
In general, when a multilayer piezoelectric element is produced, it is necessary that internal electrodes positioned between piezoelectric layers are alternately connected by an external electrode at a side surface of the element. Here, in case that the multilayer piezoelectric element is produced according to a conventional method through which a condenser type of the piezoelectric element is produced, an area of the internal electrode becomes narrower than a sectional area (surface area) of the element, thereby an electric field cannot be generated over the sectional area thereof. As a result, it is obstructed that the multilayer piezoelectric element suitably displaces according to its characteristic and displacing force is concentrated on an uneven portion in the element, thereby the multilayer piezoelectric element is apt to be destroyed.
Further, it is difficult to position and stack each of the piezoelectric layers with correct relationship thereamong when the piezoelectric layers are stacked with each other. And on the basis of difficulty in both positioning and stacking of the piezoelectric layers, there is a limit that the several tens of the piezoelectric layers can only be multilayered at best. Thereby, since the displacement quantity of the piezoelectric element proportions to a multilayered number of the piezoelectric layers when the same voltage is applied thereto, it is very difficult to produce the piezoelectric element having larger displaceability.
In order to dissolve the above problem, it is proposed a method in which the multilayer piezoelectric element is produced by multilayering the piezoelectric layers on each surface of which the internal electrode is printed thereover. Here, in the multilayer piezoelectric element produced according to the above, the area of the internal electrode on the piezoelectric layer is as same as the sectional area of the multilayer piezoelectric element. In such construction of the piezoelectric element, in order to alternately connect the internal electrodes through the external electrode, it is necessary to conduct an insulating process according to methods, for example, disclosed in U.S. Pat. No. 4,523,121 (corresponding to FIG. 13) and Japanese Patent Application Laid Open No. 62-211,974 (corresponding to FIG. 14).
Here, it will be described methods of U.S. Pat. No. 4,523,121 and Japanese Patent Application, referring to FIGS. 13 and 14. That is to say, in the electrostrictive element disclosed in FIG. 13, insulation layers 71 composed of glass are alternately formed on both side surfaces of the element by screen printing method or cataphoresis method and thereafter baking. And further, silver paste is coated on the both side surfaces of the element so as to form external electrodes 72. As a result, on each side surface of the element, internal electrodes 73 are alternately connected through the external electrodes 72.
In the laminated piezoelectric element disclosed in FIGS. 14(a) and 14(b), insulation layers 81 composed of glass are formed on a side surface of the element by screen printing method or cataphoresis method and thereafter baking and internal electrodes 83 are alternately connected to external electrodes 82 formed on the insulation layers 81 so as to partially overlap with each other.
However, as understandable from FIGS. 13 and 14, in the elements shown in FIGS. 13, 14, two additional processes are necessitated to produce the elements. The first process is to form the insulation layers 71, 81 on the side surface(s) of the elements. And the second process is to form the external electrodes 72, 82 over the insulation layers 71, 81 in order to connect both the internal electrodes 73, 83 and the external electrodes 72, 82. And further in both cases, the first process and the second process cannot be conducted at the same time since the insulation layers 71, 81 are formed and thereafter the external electrodes 72, 82 are formed, and furthermore, both the insulation layers 71, 81 and the the external electrodes 72, 82 have to be formed directly on the piezoelectric element body. Thus, there is a problem that more processes are necessary and yield for producing the piezoelectric element becomes lower.
And in case of the electrostrictive element of FIG. 13, the insulation layers 71 are alternately formed on the end portions of the internal electrodes 73, the end portions thereof being exposed on the side surfaces of the element. But, in case that the insulation layers 71 are formed by using the screen printing method, it is necessary to precisely determine printing positions of the insulation layers 71. Therefore, it will be possible that deviation of the printing positions occurs and grazing or running of the insulation layers 71 occurs. Based on such deviation, grazing or running of the insulation layers 71, there is a problem that portions which are to be connected are not efficiently connected and on the other hand, portions which are to be insulated are unnecessarily connected. Further, in case that the insulation layers 71 are formed by using the cataphoresis method, it is difficult to uniformly form the insulation layers 71 which have a thickness to be able to resist drive voltage applied to the element, therefore there is a problem that portions which are to be insulated are connected due to dielectric breakdown.
On the other hand, in case of the piezoelectric element disclosed in FIG. 14, though it is comparatively easy to form the insulation layers 81, however, it is difficult to connect the external electrodes 82 and the internal electrodes 83. For instance, if the screen printing method is utilized, it is difficult to uniformly coat the conductive paste (silver paste) on a portion having difference in level, which occurs between the side surface of the element and the surface of the insulation layers 81, in addition to the problem that it is unable to precisely determine the printing positions. Thus, there is a problem that portions which are to be connected are not efficiently connected and on the other hand, portions which are to be insulated are unnecessarily connected.
And further, even if the screen printing method or the cataphoresis method is utilized to form the insulation layers 71, 81, it is necessary an additional process to bake the insulation layers 71, 81 and the conductive paste resulting the external electrodes 72, 82. Therefore, there is a problem that producing cost of the element becomes higher and it is difficult to connect both the internal electrodes 73, 83 and the external electrodes 72, 82 if thickness of the piezoelectric layer is thinner than a thickness below 100 .mu.m.
Then, in order to dissolve the above problems, the inventors of the present invention attempted to produce the multilayer piezoelectric element by utilizing an anisotropic conductive film shown in FIG. 15, in which conductive particles are dispersed. The anisotropic conductive film is composed from an adhesive sheet 91 having a thickness of 100 .mu.m in which a plurality of conductive particles 92, each particle size being several tens .mu.m, are dispersed so as not to contact with each other. And the anisotropic conductive film is possible to have conductive portions therein, the portions having conductivity only to a thickness direction of the anisotropic conductive film, by partially pressing the anisotropic conductive film to the thickness direction thereof, thereby the conductive particles 92 are contacted with each other at the pressed portions.
By utilizing the anisotropic conductive film to produce the multilayer piezoelectric element, as mentioned above, producing process of the piezoelectric element can be simplified since it is possible to omit both the process to form the insulation layers 71, 81 and the process to bake the insulation layers 71, 81 and the conductive paste resulting the external electrodes 72, 82. Thus, producing cost of the piezoelectric element can be reduced. And connective inferiority and insulation inferiority due to grazing or running does not occur since the conductive paste or the insulation paste are not used in the process that the external electrodes 72, 82 are formed.
However, in case that the anisotropic conductive film in which the conductive particles 92 are dispersed as shown in FIG. 15 is utilized, the conductive particles 92 are apt to be contacted with each other to a direction normal to the thickness direction of the anisotropic conductive film if distances between the conductive particles 92 dispersed in the adhesive sheet 91 are narrow, when the anisotropic conductive film is partially pressed. As a result, the conductive portions are unnecessarily expanded to the direction normal to the thickness direction of the anisotropic conductive film. Therefore, if the thickness of one piezoelectric layer including the internal electrode 73, 83 becomes thinner than about 100 .mu.m, there is a problem that it is very difficult to alternately connect the internal electrodes 73, 83.